This invention relates to a dry etching method of a thin film, and in particular to a dry etching method suitable for patterning a lower wiring layer of a multi-layered wiring intersecting with an interlayer insulating film intervened therebetween. For example, this invention relates to a dry etching method of a metallic thin film for the formation of a gate wiring of thin film transistor to be employed for a liquid crystal display device.
There has been conventionally known, as a method of forming a metallic wiring on a substrate, a method of dry-etching a metallic thin film by making use of a chemical dry etching apparatus wherein a resist pattern is employed as a mask. Followings are explanations on the method of forming a wiring of molybdenum tungsten alloy (MoW) thin film on a transparent insulating substrate such as a glass substrate by making use of a conventional chemical dry etching apparatus, the explanations being made referring to the flow chart shown in FIG. 1.
First of all, a resist is coated on a thin film of the alloy on the surface of a substrate. Then, the resist is selectively exposed to light by making use of an exposure apparatus, followed by a step of developing a resist pattern using a developing apparatus, thus forming a resist mask to be employed in a subsequent etching step. Subsequently, the substrate is transferred to an etching chamber, and the etching chamber is evacuated to a high vacuum degree (for example, 3 Pa or less) (pre-etching evacuation). Thereafter, the pressure inside the etching chamber is adjusted, and then an etching gas which is required for etching the metallic thin film (such as carbon tetrafluoride and oxygen) is activated. The activated etching gas is introduced to the surface of the substrate to etch the metallic thin film formed on the substrate.
After a predetermined period of etching, the supply of the etching gas is stopped. Subsequently, the interior of the etching chamber is evacuated to a high vacuum degree (for example, 3 Pa or less), and the substrate bearing a wiring of the metallic thin film thereon is taken out of the etching chamber.
A lower wring of a multi-layered wiring intersecting with an insulating film deposited by a CVD method on the substrate easily short-circuits with an upper wiring, and the upper wiring easily opens at the edge of the lower wiring. For these reasons, the lower wiring is required to have a tapered edge surface. In particular, when the lower wiring has a thickness of at least 300 nm, it is necessary to control the shape of the lower wiring.
For example, it is required to form a wiring having a side wall with tapering angle of 30.+-.5.degree. with the surface of the substrate by selectively etching a molybdenum tungsten alloy (MoW) thin film having a thickness of 400 nm.
This tapered shape is obtained by performing the plasma discharging for 720 seconds using a mixed gas consisting of carbon tetrafluoride/oxygen gas (1/2 in mixing ratio for instance so as to realize 1/3 in ratio of etching rate between the resist and the molybdenum tungsten alloy (MoW) thin film) under the conditions of: 1,000 sccm in total etching gas flow rate, 30 Pa in pressure adjusted of the etching chamber and 800 W in microwave power.
The etching, however, takes a long period of time for the reasons explained as follows.
(1) When a metallic thin film is etched by means of dry etching method to such an extent that an underlying layer is exposed, the active radicals contributing to the etching are concentrated at a portion of the metallic thin film that still remains unetched thereby to accelerate the etching reaction of the remaining portion of the metallic thin film. Therefore, as soon as the surface of an underlying layer is exposed, the etching rate of the metallic thin film is rapidly accelerated. However, as far as the etching rate of a resist is concerned, since the etching area of the resist is not prominently changed, there is not any prominent change in etching rate thereof. As a result, the ratio in etching rate between the resist and the metallic thin film before the accomplishment of just etching of the metallic thin film is caused to change once the just etching is accomplished. PA1 (2) As mentioned above, since the time when an underlying layer begins to be exposed (hereinafter referred to as exposure-initiating time) in the process of etching is influenced by the distribution of film thickness of a metallic thin film and also by the non-uniformity in etching rate in the plane of substrate, the exposure-initiating time becomes non-uniform depending on a specific region in the plane of substrate. Due to this non-uniformity of the exposure-initiating time, a residue of redundant metallic thin film is caused to remain on the surface of substrate unless the etching is performed taking a sufficient time. On the other hand, if the etching is performed excessively, not only the tapering angle of the wiring but also the dimensional precision of pattern becomes non-uniform, thus raising another problem.
On the other hand, it is known that, in order to work the metallic thin film in such a manner that the side walls of wiring to be formed are caused to incline at a suitable tapering angle, it is required to select a suitable condition where the ratio in etching rate between the resist and the metallic thin film can be maintained at a suitable degree during the etching process.
In order to meet the aforementioned both conditions, it is required to find out a condition where the etching rate of the metallic thin film cannot be prominently changed even if the surface of underlying layer is exposed, i.e. to find out a condition where the etching rate of the metallic thin film becomes relatively slow. Further, it is required, through preliminary working of metallic thin film so as to confirm the configuration of wiring to be obtained by variously changing the etching conditions, to select etching conditions (the mixing ratio of gases, the gas flow rate, the pressure, the electric power to be applied and the etching period) which enable to obtain such a suitable ratio in etching rate between the resist and the metallic thin film that renders the side wall of wiring to become a suitable tapering angle at the finish of the etching period.
Therefore, in the conventional method, a fairly low etching rate of a metallic thin film, i.e. about 40 nm/min or less was selected, and a relatively sufficient overetchis was performed.
Thus, when a wiring is formed from a molybdenum tungsten alloy thin film and by means of the conventional etching method, it takes 10 seconds for the pre-etching evacuation, 20 seconds for the pre-etching pressure adjustment, 720 seconds for the etching and 10 seconds for the post-etching evacuation. Namely, a long period of 760 seconds in total is required for the etching treatment of a single sheet of substrate.
Meanwhile, if there is a corner portion in an electrode of a thin film transistor, an interlayer short circuit tends to be generated in a wiring, etc. being formed via an interlayer insulating film on the electrode. As a method for preventing such a short circuit, the following method has been proposed in Japanese Patent Unexamined Publication H/1-158776.
First of all, a gate electrode material is deposited on a substrate to form a gate electrode material film, which is then patterned with a resist pattern being employed as a mask thereby to carry out a first etching, thus obtaining a pattern having tapered side walls. Then, a second etching is performed under a condition where the etching rate of the resist is higher than the etching rate of the gate electrode material, thereby turning the tapered side walls into arcuate side walls.
However, since the loading effect becomes prominent if a substrate of large diameter, which is necessary to produce a liquid crystal display device, is employed in particular, it becomes very difficult to detect the changing time from the first etching to the second etching, i.e. to detect the end point of the first etching. As a result, the second etching is usually performed after the underlying layer below the gate electrode material film has been completely exposed, thus raising a problem that the width of the wiring to be formed becomes non-uniform.